An image pickup apparatus such as a digital camera often has a focusing mechanism by which the image pickup apparatus is automatically focused on a target object. For a person who uses the image pickup apparatus, adjusting a focal position of a lens while checking with the naked eye whether the image pickup apparatus is clearly focused on a target object requires an advanced skill. Thus, the focusing mechanism automating the operation is now one of the important mechanisms for the image pickup apparatus.
The focusing mechanism, however, includes components which require electric power, such as various sensors and motors. Accordingly, in the case where the focusing mechanism is always performing a focusing process by which the image pickup apparatus is focused on a target object, the image pickup apparatus consumes higher amounts of power.
In view of the foregoing, Japanese Laid-open Patent Publication No. 2006-337689 discloses a technology in which movement of the image pickup apparatus is monitored using an acceleration sensor, and the focusing process is automatically started on the basis of changes in acceleration measured by the acceleration sensor. Specifically, in the technology, when a value of the measured acceleration changes from a value larger than a predetermined threshold value to a value smaller than the predetermined threshold value, the focusing process is automatically started, assuming that the operator is ready to take a shot.
In the related technology described above, however, the focusing process may be started ahead of the time when the focusing process is actually and appropriately performed. This problem is now described with reference to FIG. 13.
FIG. 13 illustrates, by way of example, changes in acceleration measured by an acceleration sensor included in an image pickup apparatus after the image pickup apparatus has been started. Referring to FIG. 13, the period T1 illustrates a period over which an operator is about to set a composition after starting the image pickup apparatus. The period T2 illustrates a period over which the operator is about to take a shot after setting the composition. The period T3 illustrates a period over which the operator is changing the composition after completing taking of a shot. The period T4 illustrates a period over which the operator is about to take a second shot after changing the composition.
As illustrated in the example of FIG. 13, within the period (T1) over which the operator is about to set a composition after starting the image pickup apparatus, and within the period (T3) over which the operator is changing the composition, the operator moves the image pickup apparatus significantly, causing high acceleration to be measured. On the other hand, within the period (T2, T4) over which the operator is about to take a shot, the operator keeps the image pickup apparatus at rest so that a shot image is not blurred, causing only low acceleration to be measured.
Therefore, when it is configured such that the focusing process is automatically started at the time when a value of acceleration changes from a value larger than a predetermined threshold value th to a value smaller than the threshold value th, the focusing process is appropriately started upon transition from T1 to T2 and transition from T3 to T4. Such a configuration in which the focusing process is automatically started at the time when the operation to take a shot is about to be performed, as described above, is advantageous in that it properly reflects the operator's intention.
The value of acceleration, however, may also change from a value larger than a threshold value to a value smaller than the threshold value while the operator is about to set a composition or change the composition. In the example illustrated in FIG. 13, the value of acceleration changes from a value larger than the threshold value th to a value smaller than the threshold value th within the period (T1) over which the operator is about to set a composition, causing the focusing process to be started. It is undesirable that the focusing process be performed before the operator sets or changes the composition as in the above-described case, because the focusing process is performed against the operator's intention. This causes the image pickup apparatus to waste electric power.